1. Field of Invention
The present invention is directed toward a system and methods for obtaining ground conductivity information by determining an effective penetration limit of a ground penetrating radar system and associating this limit with relative ground conductivity.
2. Discussion of Related Art
Ground penetrating radar (“GPR”) systems are used to obtain measurements of subsurface structures and provide images of the internal structure of optically opaque media/materials such as soil, rock, concrete, asphalt and wood. Many GPR systems operate by radiating pulsed radio waves in the range of about 1 to 2000 megahertz (MHz) that depolarize bound electrons in the media in a way that propagates the signal through the media. The radiated pulses propagate from the system's transmitting antenna, penetrate a subsurface medium, and reflect, refract, and/or diffract at boundaries of intrinsic impedance contrasts, commonly referred to as targets, in the subsurface medium. A portion of the redirected energy propagates back to a receiving antenna, where the energy may be processed, displayed and stored. Most GPR equipment utilizes time-domain methods to process the data to construct a time versus distance profile of a series of measurements over the medium surface to provide a cross-sectional image of targets within the medium. Collecting several adjacent profile lines produces data that can be used to provide a 3-D map of the surveyed region.
A less common GPR technique utilizes stepped continuous-wave technology, which entails radiating short pulses at different frequencies. Another less commonly used GPR technique employs a continuously radiated pulse swept over a range of frequencies. The data obtained using both these methods can be converted into the equivalent time-distance map produced by time-domain GPR systems by using inverse Fourier transforms performed on each scan.
The effective penetration limit of GPR system in a medium (i.e., the maximum depth below the surface of the medium at which the GPR system can provide good data) may be constrained by electrical properties of the medium, such as, for example, the conductivity of the medium. Soil conductivities may vary significantly due to two main factors: (1) mineralogic changes, and (2) changes in the fluid composition and concentration in pores of the soil. For example, mineralogic clay, may limit the effective penetration limit of a 400 MHz center frequency GPR antenna to as little as 1 meter, whereas slightly moist sand may yield a effective penetration limit of up to 50 meters or more for a similar GPR antenna.
During the setup phase of a typical GPR survey, an operator adjusts a number of settings on the GPR control unit such as the time range, number and type of filters, and gain of the system. These settings affect the GPR data quality and may be directly influenced by the conductivity of the ground. The current approach for adjusting the settings of the GPR unit require the user to collect several test passes by moving the GPR antenna over the data collection site, visually examine the data, and then manually adjust the settings and repeat the procedure until a desirable setup is obtained. This procedure may be time-consuming and prone to operator error. In addition, in some cases, the ground conductivity may change significantly over some sections of the survey area resulting in high quality GPR data in some portions of the area and poor GPR data in others.
Variations in soil conductivity that affect GPR signal strength and the associated effective penetration limit of the GPR system can sometimes be detected and mapped using other non-invasive remote sensing techniques. For example, low-frequency electromagnetic induction (EMI) techniques are commonly used to map conductivity changes in the ground. These techniques have a number of limitations such as, for example, they have poor vertical resolution, they are very sensitive to external clutter, such as cars and atmospheric disturbances, and EMI measurements typically must be corrected for time drift.